Solar (photovoltaic) cells convert sunlight into electricity. Using an inexhaustible source of energy, the use of solar cells represents an alternative to fossil fuels that generate pollution contributing to global warning. However, in order to be economically viable, the electricity generated from solar cells must cost substantially less, either by reducing manufacturing costs or likewise by increasing efficiency.
Referring now to FIG. 1, a simplified generic photovoltaic cell is shown. Here, a solar cell is configured as an absorber with a collecting junction. In a typical silicon photovoltaic cell the absorber 103 is made up of an emitter 104 and a base region. Typically the base region is doped with atoms that create acceptor sites (e.g. boron) known as p-type silicon and the emitter region is doped with atoms that create donor sites (e.g. phosphorous) known as n-type silicon. The difference in electrostatic potential between these two regions forms the collecting junction at their interface.
In a process called photo excitation, absorbed photons of light generate electron hole pairs which are then free to move through the absorber 103 by a process of diffusion. If a diffusing carrier reaches the edge of the p-n junction it is collected and will generate current. To extract this current from the device it is necessary to include metal contacts on both the emitter and base regions. When located on the front side these metal electrodes must be patterned to allow incident light to enter the cell. In addition, the front surface of a solar cell is typically covered with a dielectric layer. This layer acts to both reduce the number of carriers lost to recombination at the front surface and to reduce the amount of light reflected from the front surface of the device. Since a dielectric does not conduct current it is necessary for the front metal electrode to contact directly to the silicon underneath this layer.
Performance of the solar cell device depends, at least in part, on the electrical contact (preferably, an ohmic contact) between electrode 105 (generally positioned above the dielectric layer) and emitter layer 104 (generally positioned below the dielectric layer). High contact resistance generally leads to a high total series resistance of the solar cell, which tends to adversely affect the fill factor (FF) and thus the overall efficiency of the solar cell. Furthermore, unwanted parasitic loss mechanisms, such as junction shunting caused by direct physical contact between the front metal and the base, will tend to result in a low junction shunt resistance which may also reduce the efficiency of the device.
To further improve the collection of carriers from within the bulk, a BSF (back surface field) layer 106 may also be added. Minimizing the impact of rear surface recombination, BSF layer 106 tends to repel those oppositely charged carriers that are located closer to the back-side. That is, the interface between BSF layer 106 and wafer absorber 103 tends to introduce a barrier to minority carrier flow to the rear surface, resulting in higher levels of minority carrier concentrations in the wafer absorber. For example, for a p-type wafer, Al (aluminum) or B (boron) may be added to repel electrons. In contrast, for an n-type wafer, P (phosphorous) may be added to repel holes. By enhancing the collection of carriers from within the bulk the efficiency of the device is improved.
The application of metal to the solar cell to create the front-side and back-side electrodes, or metallization, is generally one of the main efficiency and cost-determining steps in solar cell processing. For example, one approach of preparing front-side metal grids is to evaporate highly conductive metal layers through pre-defined shadow masks prepared, e.g., by photolithography. While yielding contact of higher quality, such approaches are not practical in the manufacturing environment where low cost and high throughput are required. An alternate approach to forming front-side metal grids includes using nickel and copper electroplated grooves, referred to as buried contacts. However, a problem with this approach is that it is also cost-intensive.
Yet a third widely used approach to front metal grids involves using screen printing technology. See, e.g., J. H. Wohlgemuth, S. Narayanan, and R. Brenneman, Proc. 21st IEEE PVSC, 221-226 (1990); J. F. Nijs, J. Sclufcik, J. Poortmans, S. Sivoththaman, and R. P. Mertens, IEEE Trans. on Elect. Dev. 46, 1948-1969 (1999). Here, the front metal grid is formed from a silver-based paste which is deposited on substrates (or material layers thereon) using a squeegee to force the paste through a screen comprised of wire mesh and a patterned emulsion. The silver-based paste typically consists of several main components, including (1) Ag particles that are typically less than about 1 μm in size, (2) a glass frit, which contains a variety of metal oxides, and (3) an organic binder.
Alternatively, the rear electrode may be formed using an Aluminum based paste which is deposited in a similar manner to the front side of the device. After deposition of both electrodes the paste is fired at a temperature of about 800° C.-900° C. During this firing process, the glass frit in the Ag based past removes the front surface dielectric layer allowing the metal to contact to the emitter layer underneath. On the rear side the firing cycle causes the Al to form a BSF layer simultaneously with electrode formation.
However, there are several disadvantages with the screen printing process described above which limit the efficiency and add complications to manufacturing. It is therefore desirable to find ways to improve this process.